This invention relates to a method of adding propulsion force to an ice breaker in the process of breaking ice in a frozen sea.
When an ice breaker navigates in a frozen sea of thick ice, continuous breaking becomes difficult. Breaking of ice is therefore usually carried out by a so-called "charging" operation. In other words, since the speed of the ship is lowered, the ship retreats and then advances to attack the ice. The hull of the ship is therefore caused to ride on the ice, so that the weight of the hull breaks the ice. However, this process requires a relatively large amount of time to break the ice, and moreover, requires the use of much fuel because of the large propulsive force that is necessary.
Various methods for solving this problem have been proposed. Two methods of notching the ice in order to easily break it are known, for example, from Japanese Utility Model Application No. 16392/1976 to Kokai. According to one of the above proposed methods, hot water or steam is injected from a nozzle provided at the bow, to form a notch in the ice. According to the other method, holes are bored in the ice by a drill provided at the bow.
A method of utilizing thermal stress is also known, for example, from Japanese Patent Application No. 1216/1980 to Kokai. According to this method, a chilling frame provided at the bow is placed on the ice, and a refrigerant such as liquid nitrogen is supplied to the frame. The ice is thereby partially chilled to form a thermal strain.
However, these methods require additional, expensive equipment. The cost of producing the thermal energy, that is, the cost of the liquid nitrogen, is also a problem.